Originally Posted by T. A. Gardner

For its weight the Churchill was. As Za points out a 2 pdr on a 40 + ton tank?!! Even in its later versions the best that could be mounted on it was a 6pdr or 75mm both of which were no better than the gun on early Shermans.

The Firefly? An AMERICAN Sherman tank modified to take a larger gun. This is not a "British" tank. It is an American tank with nothing more than a British gun shoehorned into it. Do note, the US Army Material Command did contemplate putting an M 26 turret on the Sherman (the turret rings are the same size) as an interm measure in early 1944 but didn't, instead just waiting for the M 26 to get into service.
Look at the Cromwell. To get a 17 pdr into it the British had to lenghten the hull adding an extra road wheel, and then put on a huge slab sided box of a turret ending up with the ungainly Challenger.

The Comet? An improved Cromwell that still had the same slab sided armor of the Cromwell with similar suspension (Christie vertical spring), mechanics and, the single improvement of a cut down 17pdr (the "77mm") that would fit into a properly designed turret. Some advance there.

The first really good British tank design is the Centurian, and that is virtually, if not entirely, a post war vehicle.